A polyester fiber has good mechanical properties, dimensional stability and durability, and is widely used not only for clothing but also for industrial purposes. Especially, the amount of the polyester fiber used for reinforcing rubber materials, for example, as a tire cord, is increasing, because the above characteristics are effectively utilized.
A high-strength yarn obtained by drawing a low oriented undrawn yarn at a high ratio has been used for the production of a tire cord, but the dry heat shrinkage of this high-strength yarn is high, and if the high-strength yarn is embedded as the tire cord in a rubber and a tire is formed from the tire cord-embedded rubber, the uniformity of the tire becomes poor because of the shrinkage of the cord. To avoid this disadvantage, a method has been proposed in which a relatively highly oriented undrawn yarn (pre-oriented yarn, i.e., POY) is drawn to form a high-strength yarn, and the dimensional stability of the tire cord is improved by using this high-strength yarn. This method is currently widely used for the production of tire cords.
Recently, the tendency to use a polyester fiber even in the field where rayon is used as the automobile tire cord has become marked, and a good dimensional stability not previously attainable is now required for the polyester fiber. As the technique satisfying this requirement, a method is proposed in which the spinning speed of an undrawn pre-oriented yarn (POY) is increased to improve the dimensional stability, as disclosed in Japanese Unexamined Patent Publication No. 63-165547 or Japanese Unexamined Patent Publication No. 61-19812. Although a mere increase of the spinning speed of POY in the conventional method enhances the dimensional stability, the toughness is drastically degraded with an increase of the spinning speed of POY, and furthermore, since the heat resistance (IRT) in rubber is greatly lowered, the life of a tire formed by using the obtained yarn as the tire cord is short and the durability of the tire is poor. Accordingly, the above method is not practically used.
A low shrinkage tire cord based on a similar technical idea is proposed in Japanese Unexamined Patent Publication No. 61-132616, Japanese Unexamined Patent Publication No. 61-252332 or Japanese Unexamined Patent Publication No. 62-69819, but from the results of experiments made by the inventors, it has been confirmed that the heat resistance thereof in rubber is poor, as in the case of the tire cord disclosed in Japanese Unexamined Patent Publication No. 63-165547, and in practice, to moderate this defect as disclosed in the examples of the above patent publication, a blocking agent, i.e., an agent for reducing the terminal COOH content, such as 2,2'-bis(2-oxazoline), is used in the yarn-preparing process. If this blocking agent is used, however, such disadvantages as a degradation of the yarn-preparing property, increase of fluffs, lowering of the strength and of the fatigue resistance occur, and various problems must be solved to enable a practical working of the above-mentioned method.